Medium-pressure nitrogen production with high oxygen recovery

ABSTRACT

A cryogenic air separation process for the production of oxygen and nitrogen products uses a distillation system having at least a higher pressure column and a lower pressure column. A nitrogen-enriched liquid stream is recovered from the higher pressure column and is eventually at least partially vaporized by indirect heat exchange at a pressure intermediate that of the higher pressure column and the lower pressure column. A vapor stream is withdrawn from an intermediate location of the stripping section of the lower pressure column and is at least partially condensed by indirect heat exchange with the nitrogen-enriched liquid stream. At least some of the nitrogen product is recovered from the vapor that results from the at least partial vaporization of the nitrogen-enriched liquid. The process is appropriate for the production of nitrogen in quantities up to 40 mole % of the incoming air flow.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] Not Applicable.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

[0002] Not Applicable.

BACKGROUND OF THE INVENTION

[0003] The present invention relates to cryogenic air separation processes for the production of oxygen and nitrogen products, and in particular to processes for the production of nitrogen in quantities up to 40 mole % of the incoming air flow and the production of oxygen at a purity greater than 95 mole %.

[0004] Many cryogenic air separation plants that supply oxygen as the main product also are required to produce some nitrogen. This requirement is common in various industries, such as steel production. In the well known double-column distillation configuration, nitrogen can be conveniently produced from either the lower pressure column or the higher pressure column.

[0005] Producing nitrogen as a vapor from the top of the higher pressure column decreases the amount of nitrogen that can be condensed on the top of that column and thereby decreases the amount of boilup that is produced for the lower pressure column in the thermally integrated reboiler-condenser. This action can decrease the oxygen recovery and, therefore, can increase the power consumption of that process. The increase in power consumption is due to the increased air feed flow associated with a reduction in oxygen recovery. Much of the increase in power consumption is offset by reduced nitrogen compression power, which results from recovering the nitrogen product at a pressure essentially that of the higher pressure column. If, in addition to oxygen, the plant also is producing argon, then argon recovery also decreases due to the reduced boilup in the lowest section of the lower pressure column.

[0006] If nitrogen is produced as a vapor from the top of the lower pressure column, then the boilup in the lowest section of the lower pressure column is unaffected and high levels of both oxygen and argon recovery can be maintained. However, a nitrogen product compressor typically is required to boost the nitrogen product to the desired pressure. This increases both the capital investment and the power consumption of the process. If the nitrogen requirement is relatively high (e.g., nitrogen to-oxygen product ratio of more than 1.5), this may be the preferred option, as the oxygen recovery penalty for producing nitrogen from the higher pressure column becomes more significant.

[0007] Japanese Pat. No. 07-270064 discloses a process aimed at nitrogen production from an oxygen plant with an argon column. A portion of the higher pressure column liquid nitrogen is reduced in pressure and vaporized to drive an intermediate-condenser located in the rectification section of the lower pressure column (above the point at which the oxygen-enriched stream from the higher pressure column is introduced). The nitrogen product is derived from the liquid vaporized in the intermediate-condenser. This nitrogen can be produced at a slightly higher pressure than the lower pressure column.

[0008] U.S. Pat. Nos. 4,817,394 (Erickson) and 4,854,954 (Erickson) both disclose distillation systems where nitrogen is produced at a pressure greater than the lower pressure column by vaporizing a portion of higher pressure column liquid nitrogen in an intermediate-condenser attached to the argon column.

[0009] European Patent Application No. 0 639 746 (Sweeney) discloses an air separation process where a portion of the higher pressure column liquid nitrogen is vaporized to drive a condenser on top of the lower-pressure column to generate reflux for the lower-pressure column and to produce gaseous nitrogen product. In this case, the nitrogen is produced at a pressure less than that of the lower pressure column.

[0010] It is desired to have a process having a lower pressure column and a higher pressure column whereby nitrogen is produced from the process at a pressure greater than the pressure of the lower pressure column without reducing the boilup in the bottom section of the lower pressure column.

[0011] It is further desired to have a more efficient and improved process for the production of nitrogen.

[0012] It is still further desired to have a process for the production of nitrogen which overcomes the difficulties and disadvantages of the prior art to provide better and more advantageous results.

BRIEF SUMMARY OF THE INVENTION

[0013] A modification of a typical two-column oxygen-producing cryogenic air separation process allows nitrogen production at a pressure between the pressures of the two columns by vaporizing liquid nitrogen product from the higher pressure column against a condensing lower pressure column stream. Such an arrangement increases oxygen recovery, thus reducing energy consumption.

[0014] A first embodiment of the invention is a process for separating air to produce an oxygen product and a nitrogen product. The process uses a distillation column system having at least two distillation columns, including a higher pressure column at a first pressure and a lower pressure column at a second pressure lower than the first pressure. The lower pressure column has a stripping section and is in thermal communication with the higher pressure column. Each of the distillation columns and the stripping section have a top, a bottom, and at least one intermediate location between the top and the bottom. The process includes multiple steps. The first step is to provide a stream of compressed air. The second step is to feed at least a portion of the stream of compressed air to the distillation column system. The third step is to withdraw a nitrogen-enriched liquid stream from the higher pressure column. The fourth step is to withdraw a vapor stream from the lower pressure column at an intermediate location of the stripping section of the lower pressure column. The fifth step is to heat exchange at least part of the vapor stream with at least part of the nitrogen-enriched stream, thereby at least partially condensing at least part of the vapor stream to form a condensate and at least partially vaporizing at least part of the nitrogen-enriched liquid stream to form a nitrogen-enriched vapor, at an intermediate pressure between the first pressure and the second pressure.

[0015] There are several variations of the first embodiment. In one variation, the nitrogen-enriched liquid stream is withdrawn from the top of the higher pressure column. In another variation, the nitrogen-enriched liquid is withdrawn from an intermediate location of the higher pressure column.

[0016] In another variation of the first embodiment, at least a portion of the nitrogen-enriched vapor is withdrawn from the distillation column system as at least a portion of the nitrogen product. There are several variants of this variation. In one variant, the portion of the nitrogen product that is the nitrogen-enriched vapor is less than or equal to about 40 mole % of the stream of compressed air. In another variant, the nitrogen product has a third pressure between about 1.5 times the second pressure and about 0.9 times the first pressure. In another variant, the process includes an additional step of compressing the nitrogen-enriched vapor at a temperature near an ambient temperature.

[0017] There are several additional embodiments of the invention. One such embodiment is similar to the first embodiment but includes an additional step of feeding at least a portion of the condensate to the lower pressure column at or above the intermediate location. Another embodiment also is similar to the first embodiment but includes an additional step of feeding at least a portion of the condensate to an argon distillation column at or above the bottom of the argon distillation column.

[0018] Another embodiment of the invention is a process for separating air to produce an oxygen product and a nitrogen product using a distillation column system having at least three distillation columns, including a higher pressure column at a first pressure, a lower pressure column at a second pressure lower than the first pressure, and an additional column. The lower pressure column has a stripping section and is in thermal communication with the higher pressure column. Each of the distillation columns and the stripping section have a top, a bottom, and at least one intermediate location between the top and the bottom. The process includes multiple steps. The first step is to provide a stream of compressed air. The second step is to feed at least a portion of the stream of compressed air to the distillation column system. The third step is to withdraw a nitrogen-enriched liquid stream from the higher pressure column. The fourth step is to withdraw a vapor stream from the lower pressure column at an intermediate location of the stripping section of the lower pressure column. The fifth step is to feed a first part of the vapor stream to a feed location at or adjacent the bottom of the additional column. The sixth step is to heat exchange a second part of the vapor stream with at least part of the nitrogen-enriched liquid stream, thereby at least partially condensing at least part of the second part of the vapor stream to form a condensate and at least partially vaporizing at least part of the nitrogen-enriched liquid stream to form a nitrogen-enriched vapor, at an intermediate pressure between the first pressure and the second pressure.

[0019] There are several additional embodiments of the invention which are similar to the previously discussed embodiment but include an additional step. In one such embodiment, the additional step is to feed at least a portion of the condensate to the additional column. In another embodiment, the additional step is to feed at least a portion of the condensate to the lower pressure column at or above the intermediate location.

[0020] Another aspect of the present invention is a cryogenic air separation unit using a process as in any of above described embodiments or variations thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

[0021] The invention will be described by way of example with reference to the accompanying drawings, in which:

[0022]FIG. 1 is a schematic diagram of one embodiment of the present invention;

[0023]FIG. 2 is a schematic diagram of a second embodiment of the present invention; and

[0024]FIG. 3 is a schematic diagram of a third embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0025] The present invention is a cryogenic air separation process for the production of oxygen and nitrogen. The process consists of a distillation system that comprises at least a higher pressure column and a lower pressure column. A nitrogen-enriched liquid stream is recovered from the higher pressure column and is eventually at least partially vaporized by indirect heat exchange at a pressure intermediate that of the higher pressure column and the lower pressure column. A vapor stream is withdrawn from an intermediate location of the stripping section of the lower pressure column and at least partially condensed by indirect heat exchange with the nitrogen-enriched liquid stream.

[0026] The term “stripping section” is defined to mean that section of a distillation column which resides below the location at which the feeds to that column are introduced. The associated term “intermediate location” is defined to mean a location which is not at the bottom.

[0027] The present invention is applicable for the production of oxygen at a purity greater than 95 mole % and more preferably for the production of oxygen at a purity greater than 98 mole %.

[0028] In the practice of the present invention at least some of the nitrogen product may be recovered from the vapor that results from the at least partial vaporization of the nitrogen-enriched liquid. The present invention is appropriate for the production of nitrogen in such a manner in quantities up to 40 mole % of the incoming air flow. The present invention is particularly attractive if the pressure of the nitrogen product is required to be greater than approximately 1.5 times the pressure of the lower pressure column and less than approximately 0.9 times the pressure of the higher pressure column.

[0029] The vapor stream which is withdrawn from an intermediate location of the stripping section of the lower pressure column typically contains less than 3 mole % nitrogen and less than 15 mole % argon, with the balance being primarily oxygen. After this vapor stream is at least partially condensed, it may be returned to the lower pressure column or alternatively, in the event that an argon column is associated with the process, it may be passed to the argon column.

[0030] One embodiment of the invention is shown in FIG. 1. Feed air 100 is compressed in main air compressor 102, purified in purifier 104, and divided into three streams: stream 106, stream 108, and stream 122. Stream 106 is, optionally, compressed in air booster 114, is partially cooled in main heat exchanger 112 to form stream 116, which is expanded in air expander 118, and introduced to lower pressure column 150 as feed 120. Stream 108 is partially cooled in main heat exchanger 112 and then introduced into higher pressure column 124 as feed 110. Stream 122 is increased in pressure in air booster compressor 123. The resulting pressurized stream 126 is cooled and at least partially condensed in main heat exchanger 112, and introduced to higher pressure column 124 as stream 128.

[0031] Liquid stream 130, which typically has a composition similar to that of air, is removed from an intermediate location of the higher pressure column 124 and eventually reduced in pressure across valve 131 and introduced into the lower pressure column 150 as a feed. An oxygen-enriched stream 152 is removed from the bottom of the higher pressure column and eventually reduced in pressure across valve 153 and introduced into the lower pressure column as an additional feed. Persons skilled in the art will recognize that other pressure reduction devices may be used in place of valve 153, including but not limited to dense fluid expanders and ejectors.

[0032] Higher pressure column overhead vapor stream 158 is condensed in reboiler-condenser 160 to produce liquid stream 162. The heat rejected by the condensation provides boilup for the bottom of the lower pressure column 150. A portion of liquid stream 162 provides reflux for the higher pressure column 124. Another portion, nitrogen-enriched liquid stream 164, is eventually reduced in pressure across valve 165 and at least partially vaporized in heat exchanger 192. The pressure of vaporization is intermediate the pressure of the higher pressure column and the pressure of the lower pressure column.

[0033] In this embodiment, heat exchanger 192 is enclosed in vessel 196 to facilitate the separation of vapor and liquid fractions. Heat exchanger 192 and vessel 196 may be incorporated into the higher pressure or the lower pressure column design in a vertically stacked arrangement.

[0034] That portion of the at least partially vaporized nitrogen-enriched stream which remains liquid is removed from vessel 196 as stream 170. Stream 170 is eventually reduced in pressure across valve 171 and introduced to the lower pressure column 150 as a top feed. The vapor portion from vessel 196, stream 166, is warmed in main heat exchanger 112 and recovered as medium pressure gaseous nitrogen product 168.

[0035] The heat required to at least partially vaporize the nitrogen-enriched liquid stream 164 and thereby produce vapor stream 166 is provided by at least partially condensing vapor stream 190. In accordance with the invention, stream 190 is withdrawn from a location in lower pressure column 150 below the locations at which the lower pressure column feeds enter but above the bottom reboiler-condenser 160 of the lower pressure column. In other words, vapor stream 190 is withdrawn from an intermediate location of the stripping section in the lower pressure column. Stream 190 is at least partially condensed in heat exchanger 192 to form stream 194. Stream 194 is returned to the lower pressure column at a location at or above the location from which stream 190 is withdrawn.

[0036] Liquid oxygen is removed from the lower pressure column 150 as stream 180, is raised in pressure by pump 182 to form stream 184, is vaporized and warmed in main heat exchanger 112, and is recovered as gaseous oxygen product 186. Lower pressure column vapor overhead stream 172 is a waste stream that is warmed in the main heat exchanger and eventually vented into the atmosphere as stream 176.

[0037] In the field of distillation, the condensation of a vapor stream from the stripping section of the lower pressure column (in the section below the column feeds) is counter to the conventional wisdom. Traditionally, intermediate-condensation is performed in the rectification section (in a section above the lowest feed). The present invention is able to depart from conventional practice due to the recognition that the stripping section of the lower pressure column is really performing two separations. In the lowest region of the stripping section, a relatively difficult oxygen-argon separation is taking place—a separation which requires a high level of boilup. As one moves up in the stripping section, a less difficult oxygen-nitrogen separation takes place and, consequently, less boilup is required.

[0038]FIG. 2 illustrates another embodiment of the invention. For brevity, streams in FIG. 2 common to those in FIG. 1 are omitted from the discussion below. FIG. 2 shows the application of the invention when the recovery of argon is desired.

[0039] An argon-enriched vapor is withdrawn from the stripping section of the lower pressure column 150 as stream 286. Stream 286 is split into two fractions: stream 288 and stream 290. Stream 288 is introduced to the bottom of the argon column 200. Argon-enriched stream 298 is recovered off the top of the argon column 200. Argon-depleted liquid stream 296 is returned to the lower pressure column.

[0040] As in FIG. 1, an oxygen-enriched stream 152 is removed from the bottom of the higher pressure column 124. However, in this embodiment at least a portion of stream 152 is eventually reduced in pressure across valve 153 and at least partially vaporized to form stream 252. The vaporization energy is used to provide the refrigeration necessary to drive condenser 202 of argon column 200.

[0041] In accordance with the invention, stream 290, which is derived from a vapor stream 286 taken from an intermediate location of the stripping section in the lower pressure column 150, is at least partially condensed in heat exchanger 192 to form stream 294. Stream 294 is introduced to argon column 200 at a location at or above the location to which stream 288 is introduced; alternatively stream 294 may be returned to the lower pressure column at a location at or above the location from which stream 286 is withdrawn. The refrigeration to at least partially condense stream 290 is supplied by at least partially vaporizing the nitrogen-enriched stream at an intermediate pressure thereby producing vapor stream 166.

[0042]FIG. 3 illustrates one possible nitrogen product compression arrangement. For brevity, streams in FIG. 3 common to those in FIG. 1 are omitted from the discussion below.

[0043] Referring to FIG. 3, lower pressure column vapor overhead stream 172 is eventually compressed at near ambient temperature in compressor 375 to yield nitrogen product stream 376. A portion of the higher pressure column overhead vapor stream 158 is withdrawn as high pressure nitrogen stream 300, which is compressed in compressor 309 to yield nitrogen product stream 310. The vapor portion from vessel 196, stream 166, is warmed in the main heat exchanger 112 and eventually is compressed in compressor 367 to yield nitrogen product stream 368. At least two of those three nitrogen product streams (310, 368, 376) and stream 166, and at least one compressor, may be present in an air separation unit, and the compression service may be combined in separate or integrated machines to yield one product at a certain desired pressure.

EXAMPLE

[0044] The efficacy of the present invention is best illustrated through an example. The process of FIG. 2 is selected to represent the present invention. Prior art process 1 is the process of FIG. 2 with heat exchanger 192 removed and nitrogen product recovered as a vapor from the higher pressure column 124. Prior art process 2 is the process of FIG. 2 with heat exchanger 192 removed and nitrogen product recovered as a vapor from the lower pressure column 150.

[0045] The basis of comparison is as follows: 1) oxygen product is produced at a pressure of 65 psia and purity of 99.5 mole % oxygen; 2) nitrogen product is recovered at a pressure of 35 psia, 55 psia, or 100 psia and a purity of 50 ppm oxygen—the flow of nitrogen is equal to the flow of oxygen; 3) crude argon product is recovered as a liquid at a purity of 2 mole % oxygen—the flow of argon is approximately 3% of the oxygen, this represents an argon recovery level of approximately 65%; and 4) the lower pressure column operates at approximately 18 psia at the top and the higher pressure column at approximately 72 psia at the top.

[0046] For the comparison basis defined above, only prior art process 2 requires post compression of the nitrogen product if required at 35 psia or 55 psia; all three processes require post compression of nitrogen product if required at 100 psia. The table below summarizes key results: TABLE Present Invention Prior Art 1 Prior Art 2 Oxygen Flow (lb moles/hr) 20.95 20.95 20.95 Nitrogen Flow (lb moles/hr) 20.95 20.95 20.95 Argon Flow (lb moles/hr) 0.64 0.64 0.64 Air Flow (lb moles/hr) 104.7 107.2 100.7 Compression Power (kW) Nitrogen at 35 psia 91.8 94.8 92.4 Nitrogen at 55 psia 91.8 94.8 96.8 Nitrogen at 100 psia 97.5 98.2 102.3

[0047] For producing nitrogen at 35 psia, the present invention saves 3.2% power compared to prior art process 1 and 0.7% power compared to prior art process 2. For producing nitrogen at 55 psia, the present invention saves 3.2% power compared to prior art process 1 and 5.2% power compared to prior art process 2. For producing nitrogen at 100 psia, the present invention saves 0.7% power compared to prior art-process 1 and 4.7% power compared to prior art process 2. Compression power includes the power to compress all the air feeds, plus the power to boost the nitrogen to delivery pressure (when required).

[0048] The results show that the present invention saves power over prior art process 1 even when the pressure of the nitrogen product exceeds that of the higher pressure column. The results also show that the present invention saves power over prior art process 2 when the pressure of the nitrogen product exceeds approximately twice that of the lower pressure column. Even in those cases where the power of prior art process 2 is less than that of the present invention (when nitrogen pressure is approximately less than 30 psia), the present invention has an advantage over prior art process 2 in that the costly compression machinery for the nitrogen product can be eliminated.

[0049] Numerous modifications or additions may be applied to the embodiments shown in FIGS. 1, 2 and 3.

[0050] For example, the discussion has centered around processes that produce oxygen by pumping the oxygen liquid and vaporizing it in the main heat exchanger 112. However, persons skilled in the art will recognize that the present invention also is applicable when the liquid oxygen pressure is increased without the use of a pump 128 prior to vaporization. Furthermore, the oxygen product need not be vaporized in the main heat exchanger nor is it essential that pressurized air stream 126 be at least partially condensed—some other suitable pressurized stream may be employed. In addition, the oxygen product need not be withdrawn from the lower pressure column 150 as a liquid. Rather, the oxygen may be withdrawn from the lower pressure column as a vapor, in which event pressurized air stream 126 would not be necessary.

[0051] In the preceding discussions, the liquefied air feed stream 128 is shown to be directed to the higher pressure column 124. Persons skilled in the art that will recognize some or all of the air feed stream 128 also may be sent to the lower pressure column 150. In such an event, liquid stream 130 would not be required.

[0052] Refrigeration for the process has been illustrated by the expansion of a portion of air to the lower pressure column 150. However, persons skilled in the art will recognize that other refrigeration means exist. Examples include but are not limited to: 1) expansion of a portion of air to the higher pressure column 124; 2) expansion of waste stream 172 from the lower pressure column; and 3) expansion of a vapor from the higher pressure column, such as a portion of stream 158. Furthermore, it is entirely feasible to expand vapor stream 166 in a turbo expander to provide some or all of the refrigeration required by the process.

[0053] In the discussion of the embodiments shown in FIGS. 1 and 2, reference is made to “eventually reducing in pressure”. It will be understood that this implies other processing steps may exist before the pressure reduction. For example, it is common practice to cool liquid streams prior to their introduction to the lower pressure column 150. This cooling is provided by warming cold returning vapor streams, such as stream 172.

[0054] The focus of the discussion has been on the production of a single oxygen product and a single nitrogen product. However, persons skilled in the art will recognize that the present invention also is applicable when multiple oxygen products and/or multiple nitrogen products are produced. Furthermore, these products need not be the same purity. With regard to multiple nitrogen products, one may elect to produce a portion of nitrogen from either or both of the higher pressure column 124 and/or the lower pressure column 150 in addition to the primary, intermediate pressure nitrogen product stream 168.

[0055] The reflux, or top feed, for the lower pressure column 150 is shown as stream 170. Other optional reflux streams exist. Examples include but are not limited to: 1) a liquid from an intermediate location of the higher pressure column 124; and 2) a portion of liquid stream 162 (liquid from the top of the higher pressure column). In such an event, stream 170 may or may not be optionally required.

[0056] Nitrogen-enriched liquid stream 164 is shown to be recovered from the top of the higher pressure column 124. It is within the scope of the present invention to remove stream 164 as a liquid from an intermediate location of the higher pressure column. Typically, this location will be above that point from which stream 130 is removed.

[0057] Although illustrated and described herein with reference to certain specific embodiments, the present invention is nevertheless not intended to be limited to the details shown. Rather, various modifications may be made in the details within the scope and range of equivalents of the claims and without departing from the spirit of the invention. 

1. A process for separating air to produce an oxygen product and a nitrogen product, said process using a distillation column system having at least two distillation columns, including a higher pressure column at a first pressure and a lower pressure column at a second pressure lower than the first pressure, the lower pressure column having a stripping section and being in thermal communication with the higher pressure column, wherein each distillation column and the stripping section have a top, a bottom, and at least one intermediate location between the top and the bottom, comprising the steps of: providing a stream of compressed air; feeding at least a portion of the stream of compressed air to the distillation column system; withdrawing a nitrogen-enriched liquid stream from the higher pressure column; withdrawing a vapor stream from the lower pressure column at an intermediate location of the stripping section of the lower pressure column; and heat exchanging at least part of the vapor stream with at least part of the nitrogen-enriched liquid stream, thereby at least partially condensing at least part of the vapor stream to form a condensate and at least partially vaporizing at least part of the nitrogen-enriched liquid stream to form a nitrogen-enriched vapor, at an intermediate pressure between the first pressure and the second pressure.
 2. A process as in claim 1, comprising the further step of feeding at least a portion of the condensate to the lower pressure column at or above the intermediate location.
 3. A process as in claim 1, comprising the further step of feeding at least a portion of the condensate to an argon distillation column at or above the bottom of the argon distillation column.
 4. A process as in claim 1, wherein the nitrogen-enriched liquid stream is withdrawn from the top of the higher pressure column.
 5. A process as in claim 1, wherein the nitrogen-enriched liquid is withdrawn from an intermediate location of the higher pressure column.
 6. A process as in claim 1, wherein at least a portion of the nitrogen-enriched vapor is withdrawn from the distillation column system as at least a portion of the nitrogen product.
 7. A process as in claim 6, comprising the further step of compressing said nitrogen-enriched vapor at a temperature near an ambient temperature.
 8. A process as in claim 6, wherein the portion of the nitrogen product that is the nitrogen-enriched vapor is less than or equal to about 40 mole % of the stream of compressed air.
 9. A process as in claim 6, wherein the nitrogen product has a third pressure between about 1.5 times the second pressure and about 0.9 times the first pressure.
 10. A cryogenic air separation unit using a process as in claim
 1. 11. A process for separating air to produce an oxygen product and a nitrogen product, said process using a distillation column system having at least three distillation columns, including a higher pressure column at a first pressure, a lower pressure column at a second pressure lower than the first pressure, and an additional column, the lower pressure column having a stripping section and being in thermal communication with the higher pressure column, wherein each distillation column and the stripping section have a top, a bottom, and at least one intermediate location between the top and the bottom, comprising the steps of: providing a stream of compressed air; feeding at least a portion of the stream of compressed air to the distillation column system; withdrawing a nitrogen-enriched liquid stream from the higher pressure column; withdrawing a vapor stream from the lower pressure column at an intermediate location of the stripping section of the lower pressure column; feeding a first part of the vapor stream to a feed location at or adjacent the bottom of the additional column; and heat exchanging a second part of the vapor stream with at least part of the nitrogen-enriched liquid stream, thereby at least partially condensing at least part of the second part of the vapor stream to form a condensate and at least partially vaporizing at least part of the nitrogen-enriched liquid stream to form a nitrogen-enriched vapor, at an intermediate pressure between the first pressure and the second pressure.
 12. A process as in claim 11, comprising the further step of feeding at least a portion of the condensate to the additional column.
 13. A process as in claim 11, comprising the further step of feeding at least a portion of the condensate to the lower pressure column at or above the intermediate location.
 14. A cryogenic air separation unit using a process as in claim
 11. 